1. Field of the Invention
The present invention relates to non-aqueous electrolyte solutions for electrochemical energy storage devices such as high energy density batteries and high power capacitors.
2. Discussion of the Prior Art
High voltage and high energy density rechargeable batteries based on non-aqueous electrolyte solutions are widely used as electric sources for various kinds of consumer electronic appliances, such as camcorders, notebook computers, and cell phones, because of their high voltage and high energy density as well as their reliability such as storage characteristics. This type of battery employs complexed oxides of lithium and a transition metal as positive electrode, such as LiCoO2, LiNiO2, LiMn2O4, and variations of the previous oxides with different dopants and different stoichiometry, and additionally utilizes lithium metal, lithium alloys, and carbonaceous materials as a negative electrode. Chosen over the lithium metal and lithium alloys are carbonaceous negative electrode materials, which are in general partially or fully graphitized and specially modified natural graphites. This type of battery, which uses a carbonaceous negative electrode, is also called lithium-ion (Li-ion) battery because no pure lithium metal is present in the negative electrode. During charge and discharge processes, the lithium ions are intercalated into and de-intercalated from the carbonaceous negative electrode, respectively. A significant advantage of such negative electrodes is that the problem of dendrite growth is eliminated, which is often observed in a negative electrodes of lithium metal or its alloy, and additionally prevents circuit-shorting of the cells.
Non-aqueous electrolyte solutions used in the-state-of-the-art lithium-ion batteries conventionally include a cyclic carbonate, such as ethylene carbonate (EC) or propylene carbonate (PC); and a linear carbonate, such as dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethylmethyl carbonate (EMC), and an electrolyte salt such as lithium hexafluorophosphate (LiPF6), lithium imide (LiN(SO3CF3)2), lithium trifluorosulfonate (LiCF3SO3), lithium hexafluoroarsenate (LiAsF6), and lithium tetrafluoroborate (LiBF4). The cyclic carbonates are chemically and physically stable and have high dielectric constant, which are necessary for their ability to dissolve salts. The linear carbonates are also chemically and physically stable and have low dielectric constant and low viscosity, which is required to increase the mobility of the lithium ions in the electrolytes. However, linear carbonates generally have a low boiling point and high volatility, and the cells incorporating linear carbonates can easily build up internal pressure at elevated temperatures, thereby raising safety concerns. Moreover, these linear carbonates are also highly flammable, rendering the lithium and lithium ion cells containing these components safety hazard when abused or under extreme working conditions.
As disclosed in U.S. Pat. No. 5,580,684 to Yokoyama et al. and U.S. Pat. No. 5,830,600 to Narang et al. (both of which are hereby incorporated by reference in their entirety), phosphoric acid esters of phosphorous valence V such as trimethyl phosphate and triethyl phosphate were proposed to reduce flammability of electrolyte solutions and thus to improve the safety of cells containing flammable solvents such as carbonate based solvents. However, the electrolyte solutions disclosed therein reduce flammability due to the self-extinguishing characteristic of the electrolyte. Therefore, once the electrolyte ignites, the flames are quickly eliminated as the electrolyte “burns out”.
PC-based electrolytes are those electrolyte solutions containing any PC solvent and an EC-based electrolyte for those comprising EC solvent as the only cyclic carbonate. Compared to EC, PC solvent is more oxidatively stable and has wider liquid temperature ranges. However, PC is not generally used as a solvent component in rechargeable lithium-ion batteries employing graphitic carbonaceous negative materials. This is due to the co-intercalation of PC with lithium ions into graphene layers of the graphitic carbonaceous negative materials and the further decomposition of PC between the layers or/and on the surface of the graphite. This reaction yields gases, causes exfoliation of graphitic carbonaceous negative electrode, and finally reduces the performance of lithium-ion batteries. This problem of PC decomposition must be resolved before the lithium-ion batteries can take the advantages of PC.
In terms of cost and performance, graphite is most often used as the negative electrode material for Li-ion batteries. Therefore, it is desirable to combine a graphite negative electrode and a PC-based electrolyte into a Li-ion battery, which performs in a wider temperature range and at high voltages. Coating of a protective layer onto the surface of graphite particles to prevent the co-intercalation and decomposition of PC solvents was proposed by Yoshio et al. (see J. Electrochem. Soc., 147 (4), 1245 (2000)), herein incorporated by reference in its entirety.
No matter what solvents are used for the electrolyte of Li-ion batteries, protective SEI films are formed to protect the graphite negative electrode from solvent co-intercalation and exfoliation. It has been known that the charge-discharge performance of Li-ion batteries significantly depends on the properties of these SEI films, which are closely related to the: property of the solvent. These SEI films become very resistive at temperatures below −20° C. and consequently lose the ability to protect the electrode at temperatures above 50° C. (see for example Plictha et al., “Low Temperature Electrolyte for Lithium and Lithium-Ion Batteries”, Proc. 38th Power Sources Conf., 8-11, June 1998, Cherry Hill, N.J., hereby incorporated by reference in its entirety). Therefore, it is desirable to improve electrolyte solutions for Li-ion batteries using graphite negative electrode even if those contain no PC solvent.